Accretion Disk Atmospheres

Mario Jimenez-Garate

M.I.T.
77 Massachusetts Avenue
NE80-6009, Center for Space Research
Cambridge, MA 02139

The window on high-resolution X-ray spectroscopy opened by Chandra and XMM-Newton is revealing the nature of the atmospheres and coronae of accretion disks. Observations of X-ray binaries show that dense photoionized plasmas blanket the disk. We compare accretion disk model atmospheres with observed X-ray spectra in order to derive ionization and density structure, opacity, spatial distribution, elemental composition, energetics, thermal stability,
and kinematics.

We calculated the spectrum for a disk atmosphere surrounding a supermassive Kerr black hole. I show the observable line emission signature in the soft X-rays from the flow near the horizon. This spectrum is subject to line transfer effects. Via a Monte Carlo, we find that the line ratios will be modified after repeated photoionizations and recombinations inside the atmosphere. Lines such O VIII (654 eV) and C VI (367 eV) are enhanced, while others such as Ne X (1022 eV) and O VIII RRC (871 eV) are suppressed. We predict an anti-correlation between the Fe K fluorescence and the recombination line intensities, traceable to the atmospheric structure. Our results suggest that the O VIII and N VII lines identified with XMM-Newton in MCG-6-30-15 (Branduardi-Raymont et al., 2001) are overly bright, but may be physically tenable.